This invention relates generally to a fluid-operated actuator and, more particularly, to a reciprocating hydraulically-operated actuator for shifting a utilization device. The utilization device may, for example, be an HVAC damper which is shifted in opposite directions by the actuator and under the control of a thermostat.
A typical actuator which has been used in this environment comprises a cylinder, a piston supported to move back and forth in the cylinder, and a rod attached to the piston and extending from the cylinder for connection to the utilization device. When pressure fluid such as hydraulic oil is admitted into a high pressure chamber of the cylinder, the rod is advanced to shift the utilization device in one direction. If the pressure in the chamber then is kept constant, the rod is held in a stable commanded position. When pressure in the chamber is relieved, a spring retracts the rod to effect shifting of the utilization device in the opposite direction.
The assignee of the present invention previously has sold actuators of the foregoing type and, in such an actuator, the piston carries a motor-driven pump. When the motor is energized, the pump delivers oil from a sump chamber of the cylinder through a passage in the piston, and into the high pressure chamber of the cylinder in order to advance the piston and the rod. In a proportional actuator of this type, the flow of oil to the pressure chamber is modulated in order to drive the piston to and hold the piston in a commanded position. Such an actuator requires rather complex control circuitry and particularly where there is a need for the actuator to retract automatically and completely upon loss of electrical power to the system.